Silicone gel compositions are addition reaction curable organopolysiloxane compositions comprising: organohydrogenpolysiloxane having a silicon-bonded hydrogen atom (i.e. an SiH group); organopolysiloxane having an alkenyl group such as a silicon-bonded vinyl group; and a platinum-based catalyst; and the addition reaction curable organopolysiloxane compositions provide cured materials that are gel-like and have low crosslinking density via an addition reaction of silicon-bonded hydrogen atom to the alkenyl group (e.g. Patent Documents 1 to 3). Silicone gel-cured materials cured by heating the silicone gel compositions are used for protecting electronic components, such as electronic components for vehicles and for consumer use, due to their excellent heat resistance, weather resistance, oil resistance, cold resistance, electrical insulating properties, and the like, as well as having low elastic modulus and low stress. Other elastomer products do not exhibit low elastic modulus and low stress at the same time, which is a characteristic of the silicone gel-cured materials. In recent years, heat resistance is also increasingly required in silicone gel materials formed from a silicone gel-cured material that is used for sealing due to requirements for enhancing reliability for electronic components for vehicles and for consumer use, and the like.
In a typical silicone rubber, a method in which a filler such as carbon or iron oxide is filled has been used as a method for increasing heat resistance. However, this method is not easily applicable to silicone gel materials that are required to have transparency and low viscosity since the method involving filling filler causes negative effects such as decrease in transparency, precipitation of the filler, and decrease in workability due to increased viscosity.
On the other hand, use of cerium compounds has been known for imparting heat resistance (e.g. Patent Documents 4 to 7). In particular, Patent Document 4 proposes that, by using a cerium-containing reaction product, obtained by heat-treating a siloxane composition that contains cerium compound and the like at a temperature of 150° C. or higher, in an addition reaction curable organopolysiloxane composition that provides a gel-like cured material, a silicone gel-cured material having a superior heat resistance at high temperatures compared to conventional silicone gel-cured materials can be obtained.
However, in recent years, operating temperatures of these electronic components, particularly silicone chips, have been increased to approximately 175° C., from the conventional operating temperature of approximately 150° C., along with the application in an electronic component called “power device” being more popular. In addition, operating temperature of 200° C. or greater has been increasingly demanded due to SiC semiconductors being more popular. Furthermore, compared to conventional electronic components, these electronic components have been increasingly used under severe conditions involving high temperatures as well as significant temperature differences and/or rapid changes in the temperatures. Silicone gel-cured materials obtained by these publicly known silicone gel compositions need further improvement in heat resistance when used under higher temperatures and more severe conditions than conventionally used conditions. In addition, when the conventional silicone gel-cured materials are used in, for example, power units or electronic components for vehicles used in space or a location at a high latitude where the temperature difference is significant and when the power units or electronic components are exposed to low temperatures below −40° C., there has been a problem that deterioration occurs.